We are investigating cerebral vasospasm which is an important cause of cerebral ischemia after subarachnoid hemorrhage (SAH). The long-term objective of this grant is to determine the mechanism of vasospasm after SAH and to thereby develop treatments that will prevent and/or reverse it. We have shown that hemoglobin causes vasospasm and that vasospasm is associated with impaired arterial relaxation. One mechanism of hemoglobin-induced vasospasm may be the binding and removal of nitric oxide (NO). We have used electron paramagnetic resonance (EPR) spectroscopy to detect nitrosyl hemoglobin in the subarachnoid space after SAH, proving that this mechanism occurs. We will therefore test the hypothesis that there is an NO-reversible component of vasospasm by: 1) defining the extent to which vasospasm is reversible with NO donors in a monkey model of SAH; 2) measuring heme-NO adducts (nitrosyl hemoglobin) by EPR spectroscopy in clots removed from the subarachnoid space of monkeys at different times after SAH; 3) quantifying NO in the perivascular space at different times after SAH in monkeys; and 4) defining the time course of changes in and the immunohistochemical locations of the 3 isoforms of NOS in cerebral arteries and perivascular blood clot after SAH in monkeys. Second, because vasospasm does not seem to be completely preventable by NO donors, we will investigate mechanisms of NO-independent vasospasm by: 1) measuring protein kinase G messenger ribonucleic acid, protein and activity during the time course of vasospasm in monkeys; and 2) assessing calcium sensitivity of monkey cerebral arteries during the time course of vasospasm. In a rat model, we will assess the contribution of other downstream effectors of NO-induced relaxation by: 1) assessing potassium channel function during vasospasm (calcium-activated potassium channel density, single channel conductance, and open probability will be assessed using whole cell and single channel patch clamp recordings of isolated vasospastic rat cerebrovascular smooth muscle cells); and 2) measuring whole cell calcium currents during vasospasm in rats because assessment of potassium channel function requires knowledge of intracellular calcium and because smooth muscle calcium homeostasis may be altered during vasospasm after SAH.